@Comedycook , as others have pp, most people are naturally slim, but lifestyle and diet means they aren't slim.
My slim friends and family don't want to eat between meals or eat stodgy food and huge meals, and tend to enjoy exercise. They're not all tall and slender, but the ones who aren't are shorter and lean.
I can generally eat what I like, but I am lucky in that I love salads and veg, and don't really like cakes, biscuits, bread, chips, pasta, rice, cream etc. A piece of cake or takeaway is nice occasionally, but maybe once or twice a year is fine, and I don't really want to eat if I'm not hungry I also like the buzz from doing exercise like running or weights.
PoW is tall and slim, and is obviously sporty. She'd probably be tall and slim even if she was just 'Kate next door' because she probably doesn't want to stuff herself with lots of crappy food.
I see lots of overweight children where I live. They're the ones who are walking home from school eating a magnum bar (in February!) or a packet of pringles, or walking down the high street guzzling an energy drink or shovelling down a pasty or burger.